Project Summary ? Project 3 ? Targeting the Critical Role of Membrane Transporters Mtb is an intracellular pathogen and thus is surrounded by a host cell. Because of this immersive environment, Mtb membrane proteins constitute a direct functional interface between the pathogen and its host. They sense the environment and control entry of nutrients and other molecules (including many drugs) into, and exit from the organism. Thus, they offer a ?gateway? for small molecule therapeutics. However, to date, there are only two high-resolution structures of Mtb integral membrane proteins due in part to the difficulty of expression, purification and crystallization of membrane proteins, and to the lack of focus onto Mtb. We have developed a general method to prioritize integral membrane proteins identified as viable drug targets, essential genes, or critical virulence factors in Mtb. Our priorities continue to be ranked in collaboration with members of the consortium, using preliminary drug targeting and virulence screening data. Building on our expertise developed as a national center, we have also developed robust methods to express, purify and crystallize integral membrane proteins for structure determination by X-ray crystallography, and by electron cryo-microscopy.